In most of the cases, the pieces produced by powder metallurgy need to be calibrated after the sintering step due to the dimensional variations that occur during sintering. Lubricant oil is used in the calibration to reduce friction and wear of the machine tools, as well as to facilitate extraction of the pieces from the calibration matrix. Lubricant oil is likewise usually used for storing sintered pieces and pieces produced by other manufacturing techniques. For example, refrigerant oil is also used for machining metallic pieces.
Aiming at improving the properties of the finished pieces, such as wear resistance, corrosion resistance and fatigue resistance, there are often used surface thermochemical treatments, such as nitriding, cementation, carbonitriding, etc. In order to effect these thermochemical treatments, the presence of oil on the surface and in the pores of the pieces is prejudicial, especially when the thermochemical processing is effected via plasma.
During plasma nitriding, the oil retained in the pores and on the surface of the pieces produces instabilities in the electrical discharge, contamination of the reactor, inadequate formation of the superficial layers formed (for example, by nitrates) and contamination with carbon of the material submitted to treatment by means of an inefficient cleaning. Thus, the oil must be completely removed before the thermochemical treatments of surface hardening.
In some known treating methods, the operations of cleaning and thermochemically treating the surfaces are carried out in two separate steps in distinct equipment, which requires a very long processing time, typically 20 hours, leading to low productivity and high cost.
With the purpose of obtaining a complete removal of the oil and other organic and inorganic contaminants from the metallic pieces, and also simplifying and abbreviating a subsequent phase of surface thermochemical treatment of said pieces in the same thermal cycle, there was proposed the process of surface cleaning and treatment object of Brazilian patent application PI-0105593-3, of the same applicant, according to which the pieces to be cleaned are positioned inside the plasma reactor and connected to an anode of the latter, the cathode of said reactor being connected to a negative potential. The pieces are surrounded by an ionized gas, so-called plasma, generated by an electric discharge. The electrons provoke an electronic bombardment on the pieces connected to the reactor anode. While the heat generated by the plasma, by the collision of fast ions and neutral atoms against the cathode is usually sufficient to provide vaporization of the molecularly dissociated oil, without requiring relevant changes in the plasma parameters more adequate to catalyze the reactions of interest in each cleaning phase, the heat generated by the plasma may, in many cases, not be sufficient to maintain the processing temperature required for carrying out a subsequent surface treatment on the cleaned pieces, thus being necessary to provide a resistive heating external to the reactor. Furthermore, the setting of the intensity of the electrical discharge in order to provide the temperature levels required in the subsequent surface treatment phase may cause electrical arcs in the reaction environment, causing surface damages on the pieces and contamination due to carbon deposits on the surface of the pieces, impairing further thermochemical treatments.
From the above, there were proposed the process and the plasma reactor object of the patent application PI0803774-4, also of the same applicant, describing and claiming a solution which allows to obtain, inside the reactor, homogeneous and even elevated temperatures as a function of the desired surface treatment, independently from the parameters of the electrical discharge for generating the plasma, which are more adequate to catalyze the aimed reactions in each case and without leading to the formation of electric arcs in the reaction environment.
The process and reactor proposed in the patent application PI0803774-4 allow the operations of cleaning and subsequent surface treatment of the pieces to be carried out in the same reactor, the temperature thereof being controlled by a preferably resistive external heating, allowing for a cleaning by molecular dissociation by gaseous plasma and vaporization and exhaustion of the dissociated contaminants, the interior of the plasma reactor being maintained at temperatures higher than the condensation temperature of said contaminants.
However, the phases of heating and operating the reactor, both in the cleaning operation and in the surface treatment operation, present a time span usually quite inferior to the total time span of the phases of loading and unloading the pieces and of cooling the reactor for removing or unloading the pieces. Even when carrying out a surface treatment phase after the cleaning phase, the total time span of the heating, cleaning and surface treatment will still be inferior to the total time span of the loading, unloading and cooling phases, mainly due to the cooling time if the latter is carried out without the help of an accelerated cooling system for the reactor and the reaction chamber thereof. When using an accelerated cooling system, it may be obtained a total time of loading, unloading and cooling approximately equal to the total time of heating and of operation, including at least one of the phases of cleaning and surface treatment.
Thus, the heating device and the systems for feeding ionizing gases, vacuum production and ionization, remain inoperative during the time of duration of the progressive cooling of the reactor, added of the total duration time of the loading and unloading phases of the pieces. Upon completion of the new load in the reactor, already previously cooled and unloaded, the heating device is re-activated to heat the interior of the reactor, keeping the latter in the desired temperature conditions, while also activating the systems for feeding ionizing gases, vacuum production and ionization, for carrying out at least one of the operations of cleaning and surface treatment.
The use of a heating device for each reactor presents, therefore, the drawback of keeping the heating device and the systems for feeding ionizing gases, vacuum production and ionization inoperative for a time which may correspond: to a fraction of the total heating and operation time of the reactor; to a time approximately equal to the total heating and operation time of the reactor; or also to a multiple of said total heating and operation time.
Besides the productivity loss, represented by the times of loading and unloading the pieces and of cooling the reactor, the heating device remains deactivated and cooling during the time in which it is inactive, waiting for the cooling of the reactor and for a new load of pieces in the latter, for then being activated again in order to start a new heating phase of the reactor. As mentioned above, the inactivity time of the reactor may be somewhat inferior, approximately equal or greater than the total time of heating and operation of the reactor, in order to carry out the cleaning, the surface treatment or also both operations in sequence.
It should be observed that, during the time in which the heating device remains inoperative, the systems: for feeding ionizing gases, for vacuum production and for ionization also remain inoperative, executing nothing that can be considered productive, despite the high cost they represent in an installation, in order to carry out the operations pf cleaning and/or of surface treatment of metallic pieces.